Structurally conserved Nop56/58 N-terminal domain facilitates archaeal box C/D ribonucleoprotein-guided methyltransferase activity

The roles of NOP56 in cancer and SCA36

NOP56 is a highly conserved nucleolar protein. Amplification of the intron GGCCTG hexanucleotide repeat sequence of the NOP56 gene results in spinal cerebellar ataxia type 36 (SCA36). NOP56 contains an N-terminal domain, a coiled-coil domain, and a C-terminal domain. Nucleolar protein NOP56 is significantly abnormally expressed in a number of malignant tumors, and its mechanism is different in different tumors, but its regulatory mechanism in most tumors has not been fully explored. NOP56 promotes tumorigenesis in some cancers and inhibits tumorigenesis in others. In addition, NOP56 is associated with methylation in some tumors, suggesting that NOP56 has the potential to become a tumor-specific marker. This review focuses on the structure, function, related signaling pathways, and role of NOP56 in the progression of various malignancies, and discusses the progression of NOP56 in neurodegenerative and other diseases.

A nop56 Zebrafish Loss-of-Function Model Exhibits a Severe Neurodegenerative Phenotype

NOP56 belongs to a C/D box small nucleolar ribonucleoprotein complex that is in charge of cleavage and modification of precursor ribosomal RNAs and assembly of the 60S ribosomal subunit. An intronic expansion in NOP56 gene causes Spinocerebellar Ataxia type 36, a typical late-onset autosomal dominant ataxia. Although vertebrate animal models were created for the intronic expansion, none was studied for the loss of function of NOP56. We studied a zebrafish loss-of-function model of the nop56 gene which shows 70% homology with the human gene. We observed a severe neurodegenerative phenotype in nop56 mutants, characterized mainly by absence of cerebellum, reduced numbers of spinal cord neurons, high levels of apoptosis in the central nervous system (CNS) and impaired movement, resulting in death before 7 days post-fertilization. Gene expression of genes related to C/D box complex, balance and CNS development was impaired in nop56 mutants. In our study, we characterized the first NOP56 loss-of-function vertebrate model, which is important to further understand the role of NOP56 in CNS function and development.

Discovering molecular features of intrinsically disordered regions by using evolution for contrastive learning

A major challenge to the characterization of intrinsically disordered regions (IDRs), which are widespread in the proteome, but relatively poorly understood, is the identification of molecular features that mediate functions of these regions, such as short motifs, amino acid repeats and physicochemical properties. Here, we introduce a proteome-scale feature discovery approach for IDRs. Our approach, which we call "reverse homology", exploits the principle that important functional features are conserved over evolution. We use this as a contrastive learning signal for deep learning: given a set of homologous IDRs, the neural network has to correctly choose a held-out homolog from another set of IDRs sampled randomly from the proteome. We pair reverse homology with a simple architecture and standard interpretation techniques, and show that the network learns conserved features of IDRs that can be interpreted as motifs, repeats, or bulk features like charge or amino acid propensities. We also show that our model can be used to produce visualizations of what residues and regions are most important to IDR function, generating hypotheses for uncharacterized IDRs. Our results suggest that feature discovery using unsupervised neural networks is a promising avenue to gain systematic insight into poorly understood protein sequences.

High-resolution structure of eukaryotic Fibrillarin interacting with Nop56 amino-terminal domain

Ribosomal RNA (rRNA) carries extensive 2'-O-methyl marks at functionally important sites. This simple chemical modification is thought to confer stability, promote RNA folding, and contribute to generate a heterogenous ribosome population with a yet-uncharacterized function. 2'-O-methylation occurs both in archaea and eukaryotes and is accomplished by the Box C/D RNP enzyme in an RNA-guided manner. Extensive and partially conflicting structural information exists for the archaeal enzyme, while no structural data is available for the eukaryotic enzyme. The yeast Box C/D RNP consists of a guide RNA, the RNA-primary binding protein Snu13, the two scaffold proteins Nop56 and Nop58, and the enzymatic module Nop1. Here we present the high-resolution structure of the eukaryotic Box C/D methyltransferase Nop1 from Saccharomyces cerevisiae bound to the amino-terminal domain of Nop56. We discuss similarities and differences between the interaction modes of the two proteins in archaea and eukaryotes and demonstrate that eukaryotic Nop56 recruits the methyltransferase to the Box C/D RNP through a protein-protein interface that differs substantially from the archaeal orthologs. This study represents a first achievement in understanding the evolution of the structure and function of these proteins from archaea to eukaryotes.

tRNA Modification Profiles and Codon-Decoding Strategies in Methanocaldococcus jannaschii

tRNAs play a critical role in mRNA decoding, and posttranscriptional modifications within tRNAs drive decoding efficiency and accuracy. The types and positions of tRNA modifications in model bacteria have been extensively studied, and tRNA modifications in a few eukaryotic organisms have also been characterized and localized to particular tRNA sequences. However, far less is known regarding tRNA modifications in archaea. While the identities of modifications have been determined for multiple archaeal organisms, Haloferax volcanii is the only organism for which modifications have been extensively localized to specific tRNA sequences. To improve our understanding of archaeal tRNA modification patterns and codon-decoding strategies, we have used liquid chromatography and tandem mass spectrometry to characterize and then map posttranscriptional modifications on 34 of the 35 unique tRNA sequences of Methanocaldococcus jannaschii A new posttranscriptionally modified nucleoside, 5-cyanomethyl-2-thiouridine (cnm⁵s²U), was discovered and localized to position 34. Moreover, data consistent with wyosine pathway modifications were obtained beyond the canonical tRNA^Phe as is typical for eukaryotes. The high-quality mapping of tRNA anticodon loops enriches our understanding of archaeal tRNA modification profiles and decoding strategies.IMPORTANCE While many posttranscriptional modifications in M. jannaschii tRNAs are also found in bacteria and eukaryotes, several that are unique to archaea were identified. By RNA modification mapping, the modification profiles of M. jannaschii tRNA anticodon loops were characterized, allowing a comparative analysis with H. volcanii modification profiles as well as a general comparison with bacterial and eukaryotic decoding strategies. This general comparison reveals that M. jannaschii, like H. volcanii, follows codon-decoding strategies similar to those used by bacteria, although position 37 appears to be modified to a greater extent than seen in H. volcanii.

Bcd1p controls RNA loading of the core protein Nop58 during C/D box snoRNP biogenesis

Biogenesis of eukaryotic box C/D small nucleolar ribonucleoproteins (C/D snoRNPs) is guided by conserved trans-acting factors that act collectively to assemble the core proteins SNU13/Snu13, NOP58/Nop58, NOP56/Nop56, FBL/Nop1, and box C/D small nucleolar RNAs (C/D snoRNAs), in human and in yeast, respectively. This finely elaborated process involves the sequential interplay of snoRNP-related proteins and RNA through the formation of transient pre-RNP complexes. BCD1/Bcd1 protein is essential for yeast cell growth and for the specific accumulation of box C/D snoRNAs. In this work, chromatin, RNA, and protein immunoprecipitation assays revealed the ordered loading of several snoRNP-related proteins on immature and mature snoRNA species. Our results identify Bcd1p as an assembly factor of C/D snoRNP biogenesis that is likely recruited cotranscriptionally and that directs the loading of the core protein Nop58 on RNA.

Nucleolar Division in the Promastigote Stage of Leishmania major Parasite: A Nop56 Point of View

Nucleogenesis is the cellular event responsible for the formation of the new nucleoli at the end of mitosis. This process depends on the synthesis and processing of ribosomal RNA (rRNA) and, in some eukaryotes, the transfer of nucleolar material contained in prenucleolar bodies (PNBs) to active transcription sites. The lack of a comprehensive description of the nucleolus throughout the cell cycle of the human pathogen Leishmania major prompted us to analyze the distribution of nucleolar protein 56 (Nop56) during interphase and mitosis in the promastigote stage of the parasite. By in silico analysis we show that the orthologue of Nop56 in L. major (LmNop56) contains the three characteristic Nop56 domains and that its predicted three-dimensional structure is also conserved. Fluorescence microscopy observations indicate that the nucleolar localization of LmNop56 is similar, but not identical, to that of the nucleolar protein Elp3b. Notably, unlike other nucleolar proteins, LmNop56 remains associated with the nucleolus in nonproliferative cells. Moreover, epifluorescent images indicate the preservation of the nucleolar structure throughout the closed nuclear division. Experiments performed with the related parasite Trypanosoma brucei show that nucleolar division is carried out by an analogous mechanism.

Archaeal fibrillarin-Nop5 heterodimer 2'-O-methylates RNA independently of the C/D guide RNP particle

Archaeal fibrillarin (aFib) is a well-characterized S-adenosyl methionine (SAM)-dependent RNA 2'-O-methyltransferase that is known to act in a large C/D ribonucleoprotein (RNP) complex together with Nop5 and L7Ae proteins and a box C/D guide RNA. In the reaction, the guide RNA serves to direct the methylation reaction to a specific site in tRNA or rRNA by sequence complementarity. Here we show that a Pyrococcus abyssi aFib-Nop5 heterodimer can alone perform SAM-dependent 2'-O-methylation of 16S and 23S ribosomal RNAs in vitro independently of L7Ae and C/D guide RNAs. Using tritium-labeling, mass spectrometry, and reverse transcription analysis, we identified three in vitro 2'-O-methylated positions in the 16S rRNA of P. abyssi, positions lying outside of previously reported pyrococcal C/D RNP methylation sites. This newly discovered stand-alone activity of aFib-Nop5 may provide an example of an ancestral activity retained in enzymes that were recruited to larger complexes during evolution.

Ribonucleoproteins in archaeal pre-rRNA processing and modification

Given that ribosomes are one of the most important cellular macromolecular machines, it is not surprising that there is intensive research in ribosome biogenesis. Ribosome biogenesis is a complex process. The maturation of ribosomal RNAs (rRNAs) requires not only the precise cleaving and folding of the pre-rRNA but also extensive nucleotide modifications. At the heart of the processing and modifications of pre-rRNAs in Archaea and Eukarya are ribonucleoprotein (RNP) machines. They are called small RNPs (sRNPs), in Archaea, and small nucleolar RNPs (snoRNPs), in Eukarya. Studies on ribosome biogenesis originally focused on eukaryotic systems. However, recent studies on archaeal sRNPs have provided important insights into the functions of these RNPs. This paper will introduce archaeal rRNA gene organization and pre-rRNA processing, with a particular focus on the discovery of the archaeal sRNP components, their functions in nucleotide modification, and their structures.